1. Technical Field of the Invention
This invention relates to a semiconductor device configured by using a semiconductor film having a crystal structure and concerned with a semiconductor device using field-effect transistors, particularly thin-film transistors, by the use of a crystalline semiconductor film crystally grown on an insulation surface. The invention also relates to a semiconductor device production system for crystallizing or activation of after ion implant, with the use of laser light.
2. Description of the Related Arts
There is known an art for crystallizing, by a laser process, an amorphous semiconductor film formed on a substrate of glass or the like. Laser process refers to an art for recrystallizing a damaged or amorphous layer formed on a semiconductor substrate or film, an art for crystallizing an amorphous semiconductor film formed on an insulation film, an art for improving the crystallinity of a semiconductor film having a crystal structure (crystalline semiconductor film), and so on. The laser oscillator for such laser processes uses a gas laser as represented in the excimer laser and a solid laser as represented in the YAG laser.
The use of a laser beam is characterized in that the region irradiated by a laser beam and absorbing the energy thereof can be preferentially heated up, as compared with the heating process utilizing radiation or conduction heating. For example, the laser process using an excimer laser oscillator oscillating a wavelength of 400 nm or shorter of ultraviolet light preferentially and locally heats up a semiconductor film, realizing the crystallization and activation process of a semiconductor film without causing less thermal damage to the glass substrate.
Laser processes includes those to poly-crystallize an amorphous semiconductor film without placing into a fully melt state by a high-speed scanning at a scanning speed of laser beam spot diameter×5000 per second or higher as disclosed in Patent Document 1 for example, and those to irradiate an extended laser beam to an island-formed semiconductor region to form substantially a single crystal region as disclosed in Patent Document 2 for example. Besides, there is known a method for forming the beam into a linear form by an optical system of a laser processing apparatus as disclosed in Patent Document 3.
[Patent Document 1]
JP-A-104117/1987 (page 92)
[Patent Document 2]
U.S. Pat. No. 4330,363 (FIG. 4)
[Patent Document 3]
JP-A-195357/1996 (pages 3-4, FIGS. 1-5)
Furthermore, there is disclosed an art of crystallization using a solid laser oscillator of YVO4 laser or the like in Patent Document 4, for example. In this publication, the second harmonic of laser beam emitted from the solid laser oscillator is used to obtain a crystalline semiconductor film having a greater crystal grain size as compared to the conventional, showing the application to thin film transistors (hereinafter described TFTs).
[Patent Document 4]
JP-A-2001-144027 (page 4)
Meanwhile Non-patent Document 1 also reports on such an application to the thin film transistors (hereinafter described TFTs) in an crystallization art using a solid laser oscillator. This uses the second harmonic of a diode-excited solid continuous-oscillation laser (YVO4) to crystallize an amorphous silicon film, showing a result of TFT fabrication using the same.
A. Hara, F. Takeuchi, M. Takei, K. Yoshino, K. Suga and N. Sasaki, “Ultra-high Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization”, AMLCD '01 Tech. Dig., 2001, pp.227-230.
To begin with, it has been considered, in order to improve TFT characteristics, crystallinity improvement is requisite for the active layer thereof (herein referring to the semiconductor film forming the channel region or source and drain regions).
An attempt has being made for a long time to form a single-crystal semiconductor film on an insulation surface. There is devised an art called graphoepitaxy as a more active trial. Graphoepitaxy is a technique to form a step on a surface of a quartz substrate. After forming an amorphous or polycrystal semiconductor film on that, this is heated by a laser beam or heater to form a growth layer in an epitaxial fashion by utilizing, as a nucleus, the step form formed on the quartz substrate. This art is disclosed, e.g. in Non-patent Document 2.
[Non-patent Document 2]
J. Vac. Sci. Technol., “Grapho-epitaxy of silicon on fused silica using surface micropatterns and laser crystallization”, 16(6), 1979, pp1640-1643.
Meanwhile, Non-patent Document 3 also discloses a technique of semiconductor film crystallization called graphoepitaxy. This is an attempt on epi-growth of a semiconductor film through the inducement of a surface relief grating on amorphous substrate surface artificially made. The Non-patent Document 3 discloses that the graphoepitaxy technique is to provide a step on an insulation film surface and subjecting a process of heating or laser light irradiation to the semiconductor film formed on the insulation film thereby epitaxially growing the crystal in the semiconductor film.
[Non-patent Document 3]
M. W. Geis, et al., “CRYSTALLINE SILICON ON INSULATORS BY GRAPHOEPITAXY” Technical Digest of International Electron Devices Meeting, 1979, pp.210.
However, for forming a quality crystalline semiconductor film fewer in defects or grain boundaries and aligned in orientation, particularly a single-crystal semiconductor film, on an insulation surface, the mainstream is in the method to heating a semiconductor film to a high temperature into a melt state and then recrystallize it as known by the zone melting scheme.
In the known graphoepitaxy technique, because an underlying step is utilized, a crystal grows along the step. It has been considered problematic that the step is left on a surface of a single-crystal semiconductor film formed. Also, it has been impossible to form, using graphoepitaxy, a single-crystal semiconductor film on a large-sized glass substrate comparatively low in strain point.
In any case, it has been impossible to form a crystalline semiconductor film fewer in defects, due to semiconductor volume contraction caused by crystallization, thermal stress or lattice mismatch with the underlying layer and so on. Also, position control could not be done so as to determine and form a region for accumulating strains and causing defects to an outside of a device forming region. From the above reasons, where omitting a bonded SOI (silicon on insulator), it has been impossible to obtain, by using a crystalline semiconductor film formed on an insulation surface, a quality equivalent to the MOS transistors formed on a single-crystal substrate.
The present invention has been made in view of the foregoing problems. It is an object to provide a semiconductor device to form an uniform crystalline semiconductor film, particularly preferably a single-crystal semiconductor film, on a glass substrate low in strain point thereby providing a semiconductor device configured with semiconductor elements high in operating speed and current driving capability.
Meanwhile, recently the art for forming TFTs on a substrate has greatly advanced to put forward the development for applying it to the active-matrix semiconductor device. Particularly, the TFT using a polycrystal semiconductor film has higher field-effect mobility (also called mobility) than the TFT using the conventional amorphous semiconductor film, and hence is operable at high speed. This makes it possible to implement pixel control, conventionally done by a drive circuit provided outside the substrate, by a drive circuit provided on the same substrate as the pixels.
In the meanwhile, concerning the substrate used for a semiconductor device, the glass substrate is considered hopeful rather than the single-crystal silicon substrate in respect of cost. The glass substrate is less resistive to heat and ready to be thermally deformed. Consequently, in the case polysilicon TFTs are formed on the glass substrate, the use of laser anneal for semiconductor film crystallization is much effective in avoiding thermal deformation of the glass substrate.
The features of laser anneal include great reduction in process time as compared with the anneal scheme utilizing radiation or conduction heating and less thermal damage to the substrate because of preferential, local heating the semiconductor or semiconductor film.
Incidentally, the laser anneal process herein refers to an art to recrystallize a damaged layer formed in a semiconductor substrate or film or an art to crystallize the semiconductor film formed on the substrate. Also, included is the art to be applied for planarizing or surface-quality-improving a semiconductor substrate or film. The laser oscillator applied is a gas laser oscillator as represented by the excimer laser or a solid laser oscillator as represented by the YAG laser. These are known as those for heating up a semiconductor surface layer for an extremely brief time of from approximately several tens nano-seconds to several tens micro-seconds by laser light irradiation thus causing crystallization.
The lasers are roughly divided into two, i.e. pulse oscillation and continuous oscillation, according to oscillation scheme. The pulse oscillation laser, having a comparatively high output energy, can increase producibility with a laser-beam size increased to several cm2 or greater. Particularly, in case the laser beam form is worked by using an optical system into a linear form having a length of 10 cm or greater, laser light irradiation can be effective for the substrate, thereby further increasing producibility. Consequently, in semiconductor film crystallization, the main stream has gradually been on a trend to use a pulse oscillation laser.
However, recently it has been found, in semiconductor film crystallization, that the use of a continuous oscillation laser, rather than a pulse oscillation laser, provides the greater grain size of crystal formed in a semiconductor film. With a greater crystal grain size in the semiconductor film, the TFT formed using the semiconductor film has an increased mobility to suppress TFT characteristic variation. For this reason, attentions have been abrupt drawn to the continuous oscillation laser.
The crystalline semiconductor film, formed by using a laser anneal process roughly divided into pulse and continuous oscillations, is generally formed with gathering of a plurality of crystal grains. The crystal grains are random in position and size. It is difficult to form a crystalline semiconductor film by designating a position and size of crystal grains. Consequently, there are cases that crystal grain interfaces (grain boundaries) exist in an active layer formed by patterning the crystalline semiconductor into an island form.
Unlike the inside of a crystal grain, a countless number of recombination and trap centers exist at the grain boundary that are caused by amorphous structure or crystal defects. It is known that, in case a carrier is trapped in the trap center, the potential on the grain boundary rises into a barrier against the carrier hence lowering the carrier's current transport characteristic. Accordingly, in case there is a grain boundary in the TFT active layer, particularly channel region, it has a serious effect upon the TFT characteristic, e.g. conspicuous lowering in TFT mobility, on-current decrease, or off-current increase due to current flow at the grain boundary. Meanwhile, in a plurality of TFTs fabricated on the assumption to obtain the same characteristic, there encounters characteristic variation depending upon a presence or absence of grain boundaries in the active layer.
The fact that, when irradiating laser light to a semiconductor film, the crystal grains obtained are random in position and size because of the following reason. A certain degree of time is required in causing solid phase nucleation within a liquid semiconductor film fully melted by laser light irradiation. As time elapses, a countless number of crystal nuclei occur in the fully melted region to grow crystals at the crystal nuclei. Because the position such a crystal nucleus occurs is random, crystal nuclei distribute unevenly. Since crystal growth ends where crystal grains push against one another, the crystal grains are random in position and size.
For this reason, although the channel region having an great effect upon TFT characteristic is ideally formed with a single crystal grain by excluding the influence of grain boundaries, it has been almost impossible to form an amorphous silicon film free of grain boundaries by a laser anneal scheme. Consequently, up to the present, there has never been obtained a TFT having as an active layer a crystalline silicon film crystallized by using the laser anneal process that is equivalent in characteristic to the MOS transistor fabricated on a single-crystal silicon substrate.
It is a problem of the present invention to provide, in view of the foregoing problem, a semiconductor device production system using a laser crystallization method capable of preventing grain boundaries from forming in a TFT channel region and further preventing conspicuous lowering in TFT mobility due to grain boundaries, on-current decrease or off-current increase.